Multiparameter analytical electrode structure and method of measurement

ABSTRACT

An ion sensitive electrode device and method of its use in analytical and clinical chemistry are provided for measuring the concentration of an ionizable analyte contained in an aqueous medium. The device with external circuit means includes an electrode pair adapted to be placed in electrochemical contact with the medium. The electrode pair includes a polarographically active base metal/metal oxide working electrode and a reference electrode. The external circuit means is adapted to measure the voltage generated by the electrode pair in open circuit and to measure the closed circuit current flow at a preselected impedance and including means for comparing said voltage and current flow values with predetermined reference standards to provide a third value representative of said analyte concentration.

This application is a division of application Ser. No. 027,846, filedMar. 19, 1987, now U.S. Pat. No. 4,798,655.

DESCRIPTION

1. Technical Field

This invention relates to electrochemical apparatus and methods forsensing or measuring chemical ionic species as well as dissolved gasessuch as blood gas parameters in aqueous media, physiological media, andthe like.

2. Background of the Invention

Measurement of gas pressures in aqueous fluids by polarography isconventional. Polarographic sensors are commonly used, for example, inthe monitoring of the partial pressure of dissolved oxygen (PO₂) inblood. One common form of PO₂ sensor is based on a design described byL.C. Clark (e.g., see U.S. Pat. No. 2,913,386) and includes a noblemetal cathode, a buffered electrolyte, and a reference electrode. Oxygenpresent in the electrolyte migrates to, and is electrochemically reducedat, the cathode. The magnitude of current flow resulting from an appliedpotential at the cathode is a measure of PO₂.

A more versatile apparatus for simultaneous PO₂ and partial pressure ofcarbon dioxide (PCO₂) sensing by polarography employing a single sensorthat is an inert, noble metal surface is an apparatus described byParker et al. in U.S. Pat. No. 4,452,672. The apparatus uses a referenceelectrode with the inert electrode, and one applies an externalpolarizing voltage to produce a current in the fluid sample between theelectrodes. The electrode itself does not enter into a chemicalreaction.

SUMMARY OF THE INVENTION

The present invention is based on the discovery of a novel type of basemetal-metal oxide sensor that is sensitive both to dissolved gases suchas oxygen as well as to ionized species such as H⁺ (pH) and, in thepresence of an electrolyte, exhibits a stable electrochemical potentialdifference with respect to a reference electrode. This potential isproportional to an ionic concentration parameter such as pH. Inaddition, the sensor acts as its own potential source so that anexternally applied voltage is not necessary for current flow in thefluid sample. In this case, the electrode material is not chemicallyinert and in fact enters into a chemical reaction with the analyte. Forexample, for an antimony surface:

    Sb.sub.2 O.sub.3 +6H.sup.+ +6e-⃡2Sb+3H.sub.2 O

Here the equilibrium potential is a function of the hydrogen ionconcentration (pH).

The invention in one aspect relates to an electrochemical method ofmeasuring the concentration of a given ionizable analyte contained in anaqueous solution. The method employs an electrode pair including anactive, pH-sensitive, base metal/metal oxide working electrode and areference electrode. The working electrode is of a material which isalso polarographically active (i.e., can be used in an amperometricmode) in the presence of the analyte to a degree which is a measure ofthe concentration of the analyte in the solution.

The method in a preferred embodiment comprises the steps of measuringthe open circuit voltage generated by the electrode pair in contact withthe aqueous medium, measuring a closed circuit current, which may be ashort circuit current, and comparing said voltage and current valueswith predetermined reference standards to provide values of twoparameters representative of said analyte concentrations. Thus, themethod employs a combination of potentiometric and amperometricmeasurements, (say, to determine pH and PO₂ simultaneously or in rapidsequence). The measurements and reference comparison can conveniently bedone by processing the derived signals in an electronic microprocessor.

In another preferred embodiment which is similar, the method comprisesthe steps of carrying out first and second measurements of thepolarographic current at given first and second applied or drivensteady-state voltages, respectively, and comparing the resultingobserved first and second current values with predetermined referencestandards to provide two values representative of concentrations of twodissolved gas analytes of interest for example, PO₂ and PH₂. Then, inaddition, carrying out measurement of the open circuit potential andcomparing the observed value provides a value indicative ofconcentration of an ionic species such as H⁺ (pH). The methods of theinvention, by selection of a suitable polarographic negative or positivedriving potential are applicable to the measurement of any of variousgas analytes such as oxygen, carbon dioxide, hydrogen, methane, hydrogensulfide and the like.

The invention in another aspect relates to a combination potentiometricand polarographic electrode device for measuring the concentration of anionizable analyte (such as pH) contained in an aqueous medium. Thedevice, for use with external circuit means includes an electrode pairadapted to be placed in electrochemically sensing contact with themedium, the first electrode being a polarographically activebase-metal/metal-oxide working electrode and the second being areference electrode such as a silver/silver-chloride electrode or acalomel electrode. The external circuit means which may be conventionalis adapted to measure the voltage generated by the electrode pair inopen circuit and to measure a momentary closed circuit current (e.g., apulse current for about 0.01 to 10 seconds) at a preselected impedance.The external circuit includes means for comparing said voltage andcurrent values with predetermined reference standards to provide twovalues representative of concentration of the analyte of interest.Specific details concerning the device are set forth in the descriptionwhich follows.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the drawings annexed herewith,

FIG. 1 is a diagrammatic view of a preferred embodiment of a measuringapparatus according to the invention;

FIG. 2 illustrates schematic PO₂, PH₂ and P(H₂ S) polarograms obtainedat constant pH and temperature with preferred sensor apparatus andmethods according to the invention; and

FIGS. 3 and 4 are plots illustrating respectively the pH sensitivityexpressed as output voltage and negative current flow/oxygenconcentration characteristics of the active polarographic electrodeapparatus, in a preferred embodiment according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, an aqueous fluid sample 10 containing dissolved gas(e.g., oxygen) is confined in a measuring chamber 20 which may be aflowthrough chamber. An active electrode 30 and a reference electrode 40are disposed with their sensor ends 31,41 immersed in the fluid sample10. The electrodes are connected by conductive lines 32,42 to anexternal circuit 50 including a voltage supply 51, voltmeter 52, ammeter53, and switches S₁ and S₂. It is to be understood that switches S₁ andS₂ can be microelectronic switches in the external circuit. Similarly,the voltmeter and ammeter can be microelectronic measuring devices. Thecathode 31 in a preferred form, comprises a chemically active metalsurface, preferably an antimony surface, enclosed in a insulative glassor plastic sleeve 33 open at its tip so that a polarographic sensorsmall area of metal (e.g., antimony) is exposed which naturally developsa co-extensive, corresponding metal oxide layer (not shown). Thereversible electrochemical reaction entered into with the ion beingsensed is in the case of antimony given by the equation:

    Sb.sub.2 O.sub.3 +6H.sup.+ +6e-⃡2Sb+3H.sub.2 O

E=E_(o) -0.059 pH, with E_(o) =0.145 volts for Sb relative to a standardhydrogen electrode.

The measurement of the parameters at different voltage and switchconditions is illustrated by the following tabulation:

    ______________________________________                                        Switch 1    Switch 2   Measured Parameter                                     ______________________________________                                        open        closed     open circuit                                                                  potential                                              closed      open       closed circuit                                                                current                                                ______________________________________                                    

It will be noted that the applied potential from the power supply 51 maybe varied so as to measure externally driven current flow or self drivencurrent flow, in which case the driving potential will be provided bythe electrode reaction itself.

It will further be understood that with both switch 1 and switch 2closed one measures both an applied potential and a derived currentwhose values relate to concentrations of dissolved gases or ionicspecies, and further that the number of measurements requiredcorresponds to the number of unknowns to be determined.

It is found that the active metal sensor area of the cathode 31 isadvantageously self-renewing so that it continually presents a cleanmetal surface by a natural flaking off of the metal oxide layer.Therefore it has excellent polarographic properties for catalyzing thetransfer of electrons from the tip thereby causing reliably reproducableionization of the dissolved molecular analyte. The magnitude of theactive surface area of the electrode is not critical and is subject tovariation. However, it turns out that in the polarographic mode arelatively small surface area is preferred for faster reactivity. Asensor tip with layered base metal/metal oxide other thanantimony/antimony oxide can be used. Such materials includebismuth/bismuth oxide or tantalum/tantalum oxide.

Referring to FIG. 2, the measurement of dissolved oxygen, PO₂, in anaqueous sample, according to a preferred embodiment of the invention canbe done by measuring the open circuit potential which measurementcorrelates (FIG. 3) with the sample pH and by also measuring the currentflow due to either a negative applied driving potential lying centrally(line B--B) on the oxygen plateau region P₁, or by utilizing the selfpotential of the couple formed by the electrode and its reference. Byoperating the system on the plateau, interfering effects due to pHchange are neglible. The oxygen concentration relative to current flow,as illustrated in FIG. 4, is given by the observed current flowcorresponding, by comparison with reference standards, to theappropriate PO₂ curve selected from the family of plateau curves in theP₁ region. (FIG. 2. is not drown to scale and is for illustrativepurposes only).

In similar fashion, the measurement of dissolved gas concentration for agas other than oxygen can be done. Thus, the measurement of dissolvedhydrogen, PH₂, or hydrogen sulfide P(H₂ S), respectively, can be done bymeasurement of the closed circuit current using a positive drivingpotential (line Y--Y or line Z--Z) corresponding to the family ofplateau curves in the P₂ region or the P₃ region of FIG. 2. In eachinstance, measurements of open circuit potential and current flow may beused in comparison with calibrated reference standards which dictatewhich plateau of the family of plateau curves is the appropriate measureof the true gas concentration.

The configuration of the gas sensor electrode apparatus of the inventionand its application can take any of various forms. For example, it canbe in the form of an open chamber or an enclosed chamber which may be aflowthrough chamber adapted to exclude extraneous gas components. Also,it may take the form of an intracorporeal (e.g., intravascular,periodontal, subgingival, etc.) probe having a microelectrode sensortip.

What is desired to claim as my exclusive property in the invention, isthe following:

I claim:
 1. An ion sensitive electrode device for simultaneously orsequentially measuring the concentrations of an ionizable analyte and adissolved gaseous analyte contained in an aqueous medium, comprising:I.an electrode pair including: a polarographically active base metal/metaloxide working electrode having the property of providing a stableelectrochemical potential that is a quantitative measure of said ionicspecies and acting as a catalyst for electron transfer, allowing for aquantative polarographic or amperometric measure of said dissolvedgaseous analyte; a reference electrode; II. means for supporting saidelectrodes in contact with the aqueous medium and in the same chemicalenvironment; III. external circuit means in electrical communicationwith the electrode pair including; means for measuring the voltagegenerated by the electrode pair in open circuit; means for measuring theclosed circuit current flow at a preselected impedance; and means forcomparing said voltage and current flow values with predeterminedreference standards to provide two values representative of said analyteconcentrations.
 2. A device according to claim 1 where the workingelectrode is a self-renewing base metal electrode.
 3. A device accordingto claim 1 where the working electrode is an antimony/antimony oxideelectrode.
 4. A device according to claim 1 where the referenceelectrode is a silver/silver chloride electrode or calomel electrode. 5.A device according to claim 1 where the device is a probe device adaptedfor intracorporeal sensing.
 6. An electrode device for simultaneously orsequentially measuring the concentration of a dissolved ionic speciesand a dissolved gas contained in an aqueous medium comprising:I. anelectrode pair in the same chemical environment and in electrochemicalcontact with the medium and including: a polarographically active,self-renewing base metal/metal oxide working electrode having theproperty of providing a stable electrochemical potential that is aquantitative measure of said ionic species and acting as a catalyst forelectron transfer, allowing for a quantitative polarographic oramperometric measure of said dissolved gas; and a reference electrode;II. external circuit means in electrical communication with theelectrode pair including; voltage measuring means connected so as tomeasure the potential across the electrode pair; current flow measuringmeans connected so as to measure current flow between the electrodepair; first switch means in series with the voltage measuring means forselectively placing the voltage measuring means in electricalcommunication with the electrode pair so as to measure the open circuitvoltage therebetween; second switch means in series with the currentflow measuring means for selectively placing the current flow measuringmeans in electrical communication with the pair of electrodes so as tomeasure the closed current flow therebetween; and means for comparingthe measured voltage generated by the electrode pair in open circuit andthe measured closed circuit flow at a preselected impedance with thepredetermined reference standards to provide two values representativeof the concentrations of said ionic species and dissolved gas.
 7. Thedevice of claim 6 wherein the means for comparing is a microprocessor.